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Continuing the Tracing of Human Descent: The Impact of Climate on Out-of-Africa Migration

Sediment studies in the Mediterranean reveal the existence of a green corridor in the Sahara desert that emerged at the exact time when our ancestors migrated from Africa about 2.1 million years ago

The Homo erectus tribe participates in the hunt. Illustration: depositphotos.com
The Homo erectus tribe participates in the hunt. Illustration: depositphotos.com

A green corridor in the Sahara desert emerged at the exact time when our ancestors migrated from Africa. This is according to a new study from Aarhus University.

About 6 million years ago, in the dense forests of East Africa, something spectacular happened. Chimpanzees, the closest to us in the animal kingdom, evolved in one direction, while our ancestors continued in another.

In the following millions of years, the differences between early humans and chimpanzees became greater and greater. Our ancestors came down from the trees, started walking upright on two legs, thus freeing their hands to manipulate tools.

This was the beginning of a development that ended with the conquest of most of the Earth by man.

About 2.1 million years ago, the first humans - Homo erectus - migrated from Africa. The journey went through Northeast Africa and the Middle East - areas covered mostly by desert today - and on to Europe and Asia.

A sediment core from the Mediterranean Sea

The black dot in the Mediterranean Sea shows where the sediment core was collected. This site receives terrestrial material from northeast Africa, which was influenced by the "African humid periods" that change the landscape and vegetation. Credit: Nature Communications Earth and Environment
The black dot in the Mediterranean Sea shows where the sediment core was collected. This site receives terrestrial material from Northeast Africa, which was influenced by the "African humid periods" that change the landscape and vegetation. Credit: Nature Communications Earth and Environment

For a long time, researchers tried to understand how Homo erectus could cross the dry and brutal desert, where there was no food, water or shade.

A new study from Aarhus University now suggests that Homo erectus may not have walked through the desert when it left Africa, explains Rachel Lupine, one of the researchers behind the new study.

"We know that there are repeated periods when the climate in the Sahara changes. We call the phenomenon 'Green Sahara' or 'African humid periods'. In a green period, the desert shrinks significantly and changes to a landscape similar to the savannahs we know from East Africa today," she says and continues:

"Our research shows that the Sahara, right around the time Homo erectus first migrated, was greener than at any other time in the 4.5 million years we studied. Therefore, it is very likely that they could have walked through a green corridor out of Africa." 

The species that conquered the world

Homo erectus to whom this skull belonged - was the first human species to leave Africa, about 2.1 million years ago. A new study shows that when Homo erectus migrated from Africa, the climate was rainier and the vegetation was more developed in the northeast of the continent than today. Climate cycles coexisted and created a green corridor that our ancestors probably used during their migration. Credit: Mizmareck / Flickr
Homo erectus to whom this skull belonged - was the first human species to leave Africa, about 2.1 million years ago. A new study shows that when Homo erectus migrated from Africa, the climate was rainier and the vegetation was more developed in the northeast of the continent than today. Climate cycles coexisted and created a green corridor that our ancestors probably used during their migration. Credit: Mizmareck / Flickr

The first humans of the Homo erectus species appeared more than two million years ago in East Africa.

Homo erectus was the first primitive man to learn to grind stone axes. These axes were probably used as weapons for hunting and cutting meat from his bones. They were also probably the first to learn to control fire.

Homo erectus was a little shorter than modern man, but more muscular. He had wider hips and a longer skull. In addition, he had a significantly smaller brain - about half the size of ours.

For more than 1.5 million years, Homo erectus lived and spread across the globe. From Africa to Europe, through Asia to the Straits of Malacca to many Indonesian islands. This makes Homo erectus the longest living human species. Our species, Homo sapiens, evolved only about 300,000 years ago.

The sea floor reveals the climate of the past

The Sahara, as we know it today, is in one of its driest periods. The length of such a period varies, but roughly every 20,000 years, the continent has gone through a complete cycle of both a rainy period and a dry period. These periods of rain are what Rachel Lupine called "African wet periods".

There are two cycles that also come into play. One lasted 100,000 years and the other 400,000 years. During each cycle, wetter-than-normal periods will alternate with drier-than-normal periods, says Rachel Lupine.

"But how can you really know what the climate was like in Africa hundreds of thousands of years ago? The seafloor sediments "can tell us, and we actually already know a lot about the climate of the past for this very reason, she explains."

"Using sea core samples of sediments from the bottom of the Mediterranean Sea, we can see what the climate was like millions of years ago. Sediment layers are formed on the bottom of the sea, and small molecules in these layers can tell us a lot about what the climate was like in the past."

Help from materials that make the leaves shine

Over time, new layers form on the sea floor with material coming from North Africa and overseas, where it slowly sinks to the bottom.

"In the layers there is a series of biological markers that store information about the climate of the past. One of these markers is a series of molecules that plants use to protect their leaves. They are also called cuticle - leaf wax," explains Rachel Lupine.

"Wax gives the leaves on trees, shrubs and grasses the coating that makes them shine. When plants die, most plant parts break down fairly quickly, but the wax molecules can survive for a long time. This is why we often find such molecules in sediments that are millions of years old." 

The chemical composition of the wax molecules can tell something about what the climate was like when the layer formed. For example, the hydrogen molecules in the wax can tell something about the amount of precipitation there was.

"Water contains hydrogen, so we can use hydrogen to follow the water cycle. The water on the surface of the earth contains both normal hydrogen and heavy hydrogen (deuterium). When it rains a lot, the plants are able to absorb less relatively heavy hydrogen, while when it rains less, they absorb more," she says. 

Rachel Lupine and her colleagues can tell by the amount of heavy hydrogen in the waxes of the leaves when it rained a lot and when it was dry. However, hydrogen doesn't tell you anything about which plants thrived in the humid climate, but the carbon atoms in the leaf waxes do, she explains.

The sediments that accumulate on the bottom of the sea constitute a record of the climate in the past.

"In general, there are two types of plants. We also call them C3 and C4 plants", she says and continues:

"About 90 percent of all plants are C3 plants. They thrive in most parts of the earth except in very dry or hot areas. C4 plants, on the other hand, are specially adapted to survive in areas where rain rarely falls and the temperature is high." 

Because C3 and C4 plants produce leaf waxes with different amounts of heavy carbon, researchers can tell them apart in samples. In this way, they can decipher or deduce which type of plant was more dominant at the time.

"During the migration of Homo erectus out of Africa, we found more C3 in the samples than at any other humid period in the last 4.5 million years. This is evidence that the wetter climate changed parts of the region from desert to prairie and savannah," she says.

Three types of photosynthesis

In the plant kingdom, there are - in general - three different ways to carry out photosynthesis. There are C3 and C4 plants - and a third type, called CAM plants. 

90% of all plants are C3 plants, 6% are CAM plants and only between 3 and 4 percent are C4 plants. However, not in Africa, where the great prairies have a greater proportion of C4 plants.

The difference between the plants stems from their different coping strategies when the moisture in the air and soil is limited.

When environmental conditions become too dry, C3 plants close the small pinnae in the leaves, which they use to absorb CO2. If the stomata are closed, the plant cannot perform photosynthesis and begins to burn its carbon reserves, evaporating water and CO2. If this goes on too long, the plant dies.

C4 plants, on the other hand, are able to carry out photosynthesis even in dry or arid environmental conditions. Although their bionics are closed, they continue to convert CO2 into energy. They can do this with the help of a molecule with four carbon atoms, after which the species is named.

CAM plants use a third method, and can cope in even drier areas.

Wheat, oats, rice and sunflowers are examples of C3 plants. Well-known C4 plants are corn, sugarcane, and lettuce, while succulents, cacti, and pineapples are CAM plants. Source: Khan Academy, Biology Dictionary and Lex.dk
Wheat, oats, rice and sunflowers are examples of C3 plants. Well-known C4 plants are corn, sugarcane, and lettuce, while succulents, cacti, and pineapples are CAM plants. Source: Khan Academy, Biology Dictionary and Lex.dk

The greenest period 2.1 million years ago

"The green periods in Africa occur, like the ice ages in northern latitudes, because of small changes in the Earth's orbit around the Sun. Geologists call these changes Milankovitch cycles. These two changes in particular play an important role when the Sahara receives more precipitation," explains Rachel Lupine.

"The Earth wobbles a little in its orbit around the Sun. This is the earthquake that creates climate fluctuations every 21,000 years or so - and causes the 'African humid period,'" she says and continues:

"The other reason for the fluctuations is how circular the Earth's orbit around the Sun is. During certain periods the orbit is more elliptical and at others more circular. This causes fluctuations with a frequency of approximately 400,000-100,000 years from period to period."

"The Sahara was at the peak of its green period about 2.1 million years ago. Here, some of the cycles probably overlapped to create such an environment. This corresponds to the period when Homo erectus migrated. Therefore, it is likely that the climate helped this migration." she concludes.


for the scientific article

DOI: 10.1038/s43247-023-01034-7

More of the topic in Hayadan:

4 תגובות

  1. my father
    I suggest deleting comments of:
    Ignorant, delusional, fools
    who kiss with stones
    and receive instructions from an imaginary author

  2. - The fact that the Sahara was green has been known for many years
    Mainly following the findings of rock paintings and animal remains,
    So is the fact that there is a cycle in turning the Sahara green,
    The same is true of the Milankiewicz cycle.
    - It says "the green times in Africa are happening,
    Like the ice ages in the northern latitudes',
    Inaccurate, so it is appropriate to replace "Edani" with periods
    For an era is a long duration
    And we have been in an ice age for about 35 million years
    Since then, Antarctica has "settled" on the South Pole
    And in the north, Asia closed Europe and America on the North Pole,
    which prevents the flow of water (energy) between the oceans and the poles,
    - The research verifies and adds detail to existing information,

  3. Some nonsense in one message. How pathetic.. It's hard to admit that there is a creator of the world who is the only one and there is no one else apart from him. That he created man and the whole world.. because it is mandatory.. especially if you are Jewish. Running away from the truth will not lead you to nowhere. I'm sad for you, go study Torah...True Torah!

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